1. Field of the Invention
The present invention relates to a data acquisition system, and more particularly to a system and method for acquiring data of a multi-channel superconducting quantum interference device (SQUID) signal.
2. Related Art
A SQUID (Superconducting Quantum Interference Device) is a device that can respond to a small change in magnetic field due to the quantum interference effect of magnetic flux, and is used in high-sensitive fluxmeters or biological sensors for magnetocardiography (MCG) and magnetoencephalography (MEG) measurements.
A SQUID sensor measures a fine magnetic field, and is disposed in a liquid helium Dewar placed in a magnetically shielded room (MSR) or a radio-frequency shielded room (RFSR) to perform MCG or MEG measurement on a human body.
Since a signal sensed by the SQUID sensor is a weak voltage signal, it is amplified and linearized in a flux-locked loop (FLL) circuit and transmitted out of the shield room. The signal coming out of the shield room through wires is signal-processed appropriately for MCG or MEG analysis by an analog signal processor (ASP), and then is transmitted to a data acquisition (DAQ) board and input to a computer.
FIG. 1 illustrates a system for acquiring data of a multi-channel SQUID signal using an ASP and a DAQ board.
Referring to FIG. 1, a system 100 for acquiring data of a multi-channel SQUID signal includes a SQUID sensor unit 110, an FLL circuit unit 120, conducting lines 130, an ASP 150, a filter 160, DAQ boards 170, and a computer 180.
The SQUID sensor unit 110 is disposed in a liquid helium Dewar 113 in a shield room 190, and 160 SQUID sensors operate therein. The FLL circuit unit 120 includes 10 FLL circuit modules 120-1 to 120-10. Each of the FLL circuit modules 120-1 to 120-10 includes 16 FLL circuits 121 and an output unit 123. The FLL circuits 121 amplify and linearize a signal measured by the SQUID sensor unit 110 in the shield room 190 and send the signal through the output unit 123 to the lines 130. The FLL circuits 121 are connected to the SQUID sensors respectively, and 16 FLL circuits 121 constitute one module. To drive the 160 SQUID sensors, the 10 FLL circuit modules 120-1 to 120-10, each having 16 channels, are required. The required number of lines 130 is 170, which amounts to the sum of the number of the SQUID sensors and the number of ground lines of the respective modules.
The ASP 150 is disposed in a shield room 140, and includes an analog signal processing module 151 and a direct current (DC) power supply 153. The analog signal processing module 151 includes a high-pass filter (HPF) 155, a voltage amplifier 156, a low-pass filter (LPF) 157, and a 60 Hz notch filter 158 for each channel. The ASP 150 receives the signal amplified and linearized in the FLL circuit unit 120 through the lines 130 and processes the received signal appropriately for MCG or MEG analysis. The DC power supply 153 is installed in the shield room 140 and prevents inflow of external noise.
The signal processed by the ASP 150 is sent to the plurality of DAQ boards 170 via the filter 160. The DAQ boards 170 convert the input analog signal into a digital signal and send the digital signal to the computer 180. When the 160 SQUID sensors are driven, two DAQ boards having an 80-channel voltage input or three DAQ boards having a 64-channel voltage input are used.
The computer 180 stores the digital signal or outputs a SQUID signal using application software.
In a system for acquiring data of a multi-channel SQUID signal using an ASP and DAQ board, SQUID sensors are connected to an ASP outside of a shield room containing the SQUID sensors through as many lines as the sum of the number of the SQUID sensors and the number of ground lines. This causes inflow of external noise into the shield room, and thus it is difficult to obtain accurate data. Also, the plurality of lines are combined with ground lines inside and outside the shield room to constitute a multi-loop circuit, which becomes another noise source.
Since a signal output from an FLL circuit unit has a low level, an analog signal processing system used to obtain only a required signal involves a high manufacturing cost and an increased installation area which becomes another noise source. Also, DAQ boards added to increase the number of voltage acquisition channels make loop circuits generating noise. Moreover, with increasing number of used channels, the sampling time of each channel is reduced.